Process for evaporating a solution and an evaporator for use in the process

ABSTRACT

The invention relates to a method for evaporating a solution and an evaporator applied to it. The evaporator ( 1 ) has parallel plate heat exchanger elements ( 3 ) fitted inside a jacket ( 2 ), consisting of a flexible plastic film, for example, and a liquid distribution space ( 4 ) common to the elements, from where the solution to be evaporated can be spread, through supply channels ( 6 ), on the heat transmission surfaces ( 4 ) of the elements to run from the top downwards. The solution ( 10 ) that has not evaporated on the surfaces is recycled from the bottom of the evaporator back to the liquid distribution space, and from there to the heat transmission surfaces ( 4 ) of the elements for re-evaporation. In connection with evaporation, precipitate is separated from the solution as a result of over-saturation, ending up in the recirculation flow with the solution and being separated from the solution in the liquid distribution space ( 14 ) that works as a separator for the precipitate. The recirculation flow is fed into the space ( 14 ) so that the precipitate in it is separated under the effect of its weight and/or kinetic energy, while the flow of the solution is directed upwards, ending up in the supply channels ( 6 ) leading to the heat transmission surfaces ( 4 ) of the elements. The space ( 14 ) can consist of an elongated duct, the flow being fed to its end from a downward curving recirculation line, or the space can consist of a trough, which is provided with lamellas ( 16 ) or a perforated intermediate bottom, which separate the precipitate.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FI00/00278 which has an Internationalfiling date of Mar. 31, 2000, which designated the United States ofAmerica.

The object of the invention is a method for evaporating a solution,comprising the spreading of the solution on the heat transmissionsurfaces of the parallel, plate heat exchanger elements of an evaporatorto run from the top downwards, the solution being fed from a liquiddistribution space common to both elements; the solution that remains onthe heat transmission surfaces without evaporating and the precipitatethat is formed in connection with evaporation are removed from the lowerend of the evaporator, and the solution that has not evaporated isrecycled back to the heat transmission surfaces for re-evaporation.Furthermore, the invention is directed at the evaporator used in thesaid method.

The publications FI 79,948 and 86,961 describe heat exchangers made ofbag-like heat transmission elements consisting of film material, such asplastic, which are suitable, among others, for distillation and forconcentrating various suspensions. In the heat exchanger, the elementsare tied against one another to form a pack, in which water is lead tothe outer surfaces of the elements to be evaporated, and then theevaporated steam is compressed to a higher pressure and temperature by acompressor and conducted inside the elements to constitute heatingsteam, which in the heat transmission is condensed back into water.

The degree of saturation of the components dissolved in theconcentration of solutions by evaporation grows, and when the saturationpoint is exceeded, precipitation results. As examples, we could mentionthe calcium oxalate precipitated from the bleaching effluents ofchemical pulp, the calcium carbonate, calcium sulphate, and calciumsilicate, as well as possible iron compounds precipitated from subsoilwaters, the denaturised proteins precipitated from the waster water ofthe food industry, and salts such as gypsum and iron salts or hydroxidesprecipitated from mineral-bearing waste water. In the heat exchangersaccording to the publications mentioned above, the precipitate formed onthe film surfaces, as well as the solid matter contained by thesuspensions that are treated, are easily accumulated into the form of acake between the bag-like elements, impeding heat transmission and theflow of liquid and steam, which is why the gaps between the elementsmust perhaps be cleaned from time to time. However, the FI applicationNo. 970,273 discloses an evaporator with improved shapes of elements, sothat, during evaporation, the precipitate or other solid matter fallfrom between the elements onto the bottom of the evaporator; in otherwords, regarding the elements, the evaporator is self-cleaning.

In evaporators, where the portion of the treated solution or suspensionthat has not evaporated is recycled back onto the heat transmissionsurfaces to achieve a sufficient degree of evaporation, one problemremains: the solid matter falling from between the elements onto thebottom of the evaporator gets into the liquid circulation flow, possiblyblocking the narrow liquid distribution channels at the upper ends ofthe elements, from where liquid is fed onto the surfaces of theelements. As the efficiency of evaporation is crucially dependent on aneven spreading of liquid onto the heat transmission surfaces of theelements, the precipitate and other solid matter must be removed fromthe circulation flow in order to prevent blockages in the feedingchannels.

The problem with blocking could be alleviated by simply providing thecirculation line with a separation device, such as a filter, a cyclone,or a sedimentator, which would separate the precipitate from the liquidbefore it is recycled back to the evaporation phase, as mentioned above.However, from the point of view of space utilization and costs, thissolution would be disadvantageous; in addition, the pressure loss causedby the separator increases the use of energy needed for pumping. If theseparator is located at the suction face of the circulation pump, thepressure loss can cause cavitation of the pump. Furthermore, the solidmatter coming off from the walls of the recycling tube system subsequentto the separator, which would end up in the liquid distribution channelsof the elements, remains a problem.

To avoid the disadvantages mentioned above, according to the invention,the separation of the precipitate or other solid matter from thesolution recycled to re-evaporation is arranged so as to take place inconnection with the distribution of the liquid to the feeding flowleading to the heat transmission surfaces of the various elements of theevaporator. The method according to the invention is characterized inthat the recycled solution is fed to the liquid distribution space sothat the precipitate in the solution is separated in the space under theeffect of its weight and/or kinetic energy at the same time as the flowof the solution is directed upwards, that the precipitate is removed tothe exhaust pipe that starts from the bottom of the space, and that thesolution is conducted from the space to the feeding units leading to theheat transmission surfaces of the elements.

The invention is suitable for film evaporators in particular, in whichbag-like heat exchanger elements consist of flexible film material, suchas plastic film. In these, the precipitate can come off from the heattransmission surfaces not only in connection with washing, but alsoduring a run; in other words, they can be self-cleaning, so that it isessential to remove the loosened precipitate from the solutioncirculation flow.

According to the invention, by connecting the separation of precipitateto the solution feeding that goes to the heat transmission surfaces itis possible to remove, from the solution, the solid matter originatingin not only the heat transmission surfaces but also the recycling tubesystems, just before the feeding phase, which is the most crucial phasewith regard to blocking. The separation of the precipitate thus arrangeddoes not impede the washing of the evaporator, where large amounts ofloosening precipitate go to the wash water, which is removed from thebottom of the evaporator. With respect to the utilization of space andthe functionality, it is preferable to locate the liquid distributionspace inside the evaporator jacket.

The liquid distribution space can preferably be designed as an elongatedduct, one end of which is connected to the recirculation line of thesolution, and the opposite end is provided with an exhaust pipe for theprecipitate. In this solution, the feeding units leading to the heattransmission surfaces are preferably distributive nozzles that beginfrom the liquid distribution space and spread out like fans, and eachone of them feeds solution to several parallel gaps between the heattransmission surfaces of the heat exchanger elements, where evaporationtakes place. Before joining the liquid distribution space, therecirculation line preferably forms a curve directed towards the spacedownwards from above, which causes the centrifugal force to press theprecipitate to the circumference of the line and to the bottom of theliquid distribution space, which is its extension, already at the stagewhen the solution is coming. The precipitate then drifts, in the form ofa bottom flow, along the shortest route from the space to the exhaustpipe.

Alternatively, the liquid distribution space can consist of an elongatedtrough, which can be provided with parallel, slanting lamellas, underwhich the recycled solution is fed and between which the solution canflow upwards. In that case, the flow of the solution winds into the flowchannels between the lamellas, which are directed upwards, while theprecipitate at the same time is separated from the flow under the effectof centrifugal force. This separation based on the kinetic energy of theprecipitate is effective especially, when the lamellas are slopedupstream with respect to the incoming direction of the circulated flow.The said curvature of the recirculation line of the solution isadvantageous also in this application.

In addition to or instead of the kinetic energy of the precipitateparticles, gravitational force can be utilized in the separation of theprecipitate by arranging laminar flowing conditions in the liquiddistribution space so that the space with its slanting lamellas works asa lamellate settling apparatus. The sedimentation of the particles isadvanced, if the bottom of the liquid distribution space is downwardsslanting in the incoming direction of the circulated flow.

Furthermore, it is preferable to design the liquid distribution space orits lower part so that it converges, in the incoming direction of thecirculated flow, in a sphenoid or conic form towards the exhaust pipethat starts from the opposite side of the space to the recirculationline. In that case, the speed of the stream flow can be kept essentiallystable so that, in the space, an even upward flow and an evendistribution of liquid to the feeding units of the various heattransmission elements is provided.

Instead of the said slanting lamellas, the trough-like liquiddistribution space can be provided with an intermediate bottom thatdivides it into a lower and upper part, comprising the necessary portsfor up flow. The ports can be slanting and the walls defining them canhave a more or less lamella-type shape to enhance the separation of theprecipitate, or the intermediate bottom can have separating members thatpermeate the flow, such as cyclones or slanting or curved pipes thatserve as flow channels.

The precipitate, which is separated from the liquid distribution spaceto the exhaust pipe, can be lead to a clarifier, where the precipitateis separated from the liquid that comes with it, the amount of theliquid generally being about 3–50%, preferably 3–25%, of the totalamount of the flow circulated in the evaporator, whereupon the liquidcan be returned to the recycled flow.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described in detail with the aid ofexamples and with reference to the appended drawings, in which:

FIG. 1 shows a cross section of an evaporator according to theinvention, comprising heat transmission elements made of film material,and liquid circulation channels that have the separation of solid matterarranged in them,

FIG. 2 shows the liquid distribution trough of the evaporator in sectionII—II of FIG. 1,

FIG. 3 shows, like FIG. 2, the liquid distribution trough according toanother embodiment of the invention,

FIG. 4 is the horizontal section IV—IV of FIG. 3,

FIG. 5 shows the lower part and the intermediate bottom, provided withprecipitate separation members, of the liquid distribution trough inaccordance with a third embodiment of the invention,

FIG. 6 shows a fifth embodiment of the invention, where paralleldistributive nozzles are connected to a tubular liquid distributionspace to feed liquid to the heat transmission surfaces of the elements,and

FIG. 7 is the section VII—VII of the pipe and the distributive nozzleaccording to FIG. 6.

Evaporator 1 according to FIG. 1 comprises a cylindrical jacket 2 andparallel, bag-like heat transmission elements 3 made of plastic film andlocated inside the jacket. In the evaporator, elements 3 are tied into apack that can consist of several dozens of elements. The evaporation byheat of the solution that is treated takes places on the outer surfaces4 of the elements; in other words, in the gaps between the elementslocated against one another. Heat is obtained from the steam that issimultaneously condensed inside the elements. The steam generated by theevaporation can be used as heating steam and it is circulated through acompressor to the supply channels of steam (not shown) leading insidethe elements.

The upper end of each bag-like heat transmission element 3 comprises alath 5 that is suitably cast from plastic, containing channels 6 forfeeding the liquid to be evaporated to the film surfaces between theelements to run downwards from above. By using vertical, winding joints7, the interior of element 3 is divided into channels that direct theflow of the heating steam and the condensate generated by it towards adiscoidal condensate eliminator 8 located at the lower end of theelement and jointed inside the element. Bottoms 9 of adjacent elements 3on both sides of condensate eliminator 8 remain sufficiently apart fromone another, so that they allow the precipitate, which was formed in thegaps between the elements in connection with the evaporation or othersolid matter that came along with the solution that was evaporated, tofall onto the bottom of the evaporator, where also solution 10 that didnot evaporate accumulates.

As at each time of evaporation only a small portion of the solution tobe evaporated is converted into steam, evaporator 1 comprises equipmentthat can be used to repeatedly recycle the solution that has notevaporated back to film surfaces 4 of the elements for re-evaporation.The equipment in question consists of recirculation line 11 that startsfrom the bottom of the evaporator, combined with line 12, which bringsnew solution to be evaporated in the evaporation process, pump 13,interior liquid distribution trough 14 of evaporator jacket 2, dam plate15 that is located in the trough and works as an overflow threshold, andthe supply channels 6 of liquid at the upper ends of the elements thatwe already mentioned. The purpose of the liquid distribution trough 14is to provide as even a distribution of the solution fed to theevaporation as possible between channels 6 belonging to various elements3. The solution is supplied onto the film surfaces 4 of the elementssymmetrically from the liquid distribution troughs 14 on both sides ofthe elements, of which, however, only one is shown in detail in FIG. 1.

FIG. 2 illustrates best the structure of liquid distribution trough 14,which, according to the invention, also works as the separator of theprecipitate or other solid matter that comes with the recycled solution.Trough 14 is provided with a number of parallel, slanting lamellas 16,which divide the trough into a lower and upper part 17, 18. According tothe figure, inlet conduit 11 for the solution, which is downwardscurved, joins with the lower part 17 of the trough, the bottom 19 ofwhich slants towards exhaust piping 21 for the precipitate that startsfrom the opposite side to the mouth 20 of the circulation line of thetrough. Parallel lamellas 16 are slanted towards the incoming directionof the solution so that, in accordance with the arrows in FIG. 2, theflow must wind more than 90° in order to get to flow channels 22 betweenthe lamellas, which are directed obliquely upwards. In this condition,solid matter 23, which comes with the solution, can be separated fromthe stream flow partly under the effect of its own kinetic energy, i.e.,centrifugal force, and partly under the effect of gravitational force,and allowed to sediment towards exhaust piping 21 that starts from thebottom of the trough. By adjusting the rate of flow, the process of flowcan be kept, in a laminar and sufficiently slow state, in the lower part17 of the trough, in the gaps between lamellas 16, so that lamellas 16,which work like clarifiers, ultimately prevent the solid matter fromgetting to the upper part 18 of the trough, at least not to an adverseextent. In the upper part of the trough, dam plate 15 converts thestream flow, which goes into supply channels 6, into a turbulent form,further decreasing the risk of blocking in the narrow supply channels 6that are divided into numerous branches (cf. FIG. 1).

In addition to the precipitate, liquid is removed from liquiddistribution trough 14 into pipe 21; the amount of the liquid can varywithin 3–50% of the flow coming to the trough through recirculation line11. According to FIG. 1, the final separation of the precipitate fromsolid matter takes place in lamellate settling apparatus 24, from wherethe precipitate is removed into line 25 and the liquid is returnedthrough line 26 to the suction face of circulation pump 13. From time totime, precipitate can be removed by rinsing with the valves of lines 21and 26 being closed.

FIGS. 3 and 4 show liquid distribution trough 14 of the evaporator,which differs from the one in FIG. 2 in that the trough has a flatbottom but it narrows in a V shape from mouth 20 of the recirculationline towards the opposite side of the trough, and that instead ofslanting lamellas, the trough comprises intermediate bottom 27comprising crooked pieces of pipe 28 that work as precipitateseparators, allowing liquid to flow through. Gravitational force and thecentrifugal force acting in the curved inlet conduit 11 press theprecipitate towards the outer circumference of the curve and the bottomof trough 14, so that the majority of the precipitate drift directly toexhaust pipe 21 under the effect of its kinetic energy. The stream flowis directed to the said precipitate separators, where gravitationalforce separates the precipitate remaining in the flow, while the streamflow continues, through the lateral openings 29 at the upper ends of theseparators, to the upper part 18 of liquid distribution trough 14. Theflow rate in all pieces of pipe 28 is essentially the same because ofthe narrowing shape of trough 14.

In the application of liquid distribution trough 14 shown in FIG. 5, thecrooked pieces of pipe 28 according to FIG. 3 are replaced with L-shapedprojections 30 bordering the flow-through openings in intermediatebottom 27. Otherwise, the application in FIG. 5 corresponds to what isdescribed above.

FIGS. 6 and 7 show an application of the invention, where liquiddistribution space 14 consists of a pipe with an essentially roundcross-section, which is an extension of inlet conduit 11. According toFIG. 7, pipe 11 forms a curve, where centrifugal force presses the solidmatter contained by the liquid to the outer circumference of the curve,and further to the bottom of liquid distribution space 14, from wherethe solid matter ends up in exhaust pipe 21. Parallel distributivenozzles 31 are attached to liquid distribution space 14, distributingthe liquid, which is mainly purified of solid matter, to liquid channels6 contained by end laths 5 of the parallel heat transmission elements 3.Tips 32 of distributive nozzles 31 extending inside liquid distributionspace 14 are bevelled to form an angle α, which is suitably about10–35°, and the nozzles expand in a fan-like shape, so that each one ofthem feeds liquid to several adjacent elements 3. Furthermore,distributive nozzles 31 are provided with inner baffle plates 33 toensure an even distribution of liquid.

It is obvious to those skilled in the art that the various embodimentsof the invention are not limited to the examples described above, butcan vary within the following claims. Thus, the separation ofprecipitate according to the invention can be applied not only in thefilm evaporators described above but also in traditional evaporatorscomprising metal heat transmission elements.

1. A method of evaporating a solution, comprising feeding the solutionto heat transmission surfaces (4) of parallel plate-formed heatexchanger elements (3) of an evaporator (1), from supply units (6, 31)spreading the solution to the top of said surfaces to flow downwards,removing the part of the solution (10) remaining from the evaporationand precipitate formed in connection with the evaporation from the lowerend of the evaporator, and recycling said remaining part of the solution(10) back to the heat transmission surfaces (4) for re-evaporation, saidrecycling comprising conducting the solution to a liquid distributionspace (14) common to said heat exchanger elements (3), separating theprecipitate (23) from the solution in said distribution space, thesolution forming an upward flow in the distribution space, and passingthe solution to said supply units (6, 31) for being spread onto the heattransmission surfaces (4), wherein the recycled solution is fed to theliquid distribution space (14) from a downwardly curved conduit (11) asa curved flow, to separate the precipitate (23) under the combinedeffect of gravity and centrifugal force, and the precipitate asseparated is discharged to an exhaust pipe (21) from the bottom of theliquid distribution space.
 2. The method according to claim 1, whereinthe solution to be recycled is fed into a narrow, elongated liquiddistribution space (14) from its one end, and that the precipitate isremoved into an exhaust pipe from the opposite end of the space.
 3. Themethod according to claim 1 or 2, wherein the solution to be recycled isfed underneath parallel lamellas (16) or an intermediate bottom (27)provided with ports (28–30), which are located in the liquiddistribution space (14), so that the flow channels (22) between thelamellas, and the precipitate (23) is separated from the flow under theeffect of centrifugal force.
 4. The method according to claim 1 or 2,wherein the precipitate is lead through an exhaust pipe (21) to asettling apparatus (25), where the precipitate is separated from theliquid phase the comes with it, after which the liquid phase isconnected to the recirculation flow of the solution that takes place inthe evaporator.
 5. The method according to claim 1 or 2, wherein theevaporator is a film evaporator consisting of heat exchanger elements(3) made of flexible film material.
 6. An evaporator (1) comprising ajacket (2), parallel upright plate heat exchanger elements (3) fittedinside the jacket, said elements having upright heat transmissionsurfaces (4), supply units (6, 31) for spreading a solution to beevaporated to the top of said heat transmission surfaces to flowdownwards on said surfaces, a liquid distribution space (14) common tosaid heat exchanger elements for feeding the solution to said supplyunits, means for removing the part of the solution (10) remaining fromthe evaporation and precipitate formed in connection with theevaporation from the lower end of the evaporator and for recycling saidremaining part of the solution (10) back to the heat transmissionsurfaces (4) for re-evaporation, said recycling means comprising aconduit (11) connecting said lower end of the evaporator with saidliquid distribution space (14), said space having means for separatingthe precipitate (23) from the solution being recycled, wherein saidconduit (11) for recycling the solution forms a downward curve connectedto the liquid distribution space (14), to feed the solution to saidspace as a curved flow and to separate the precipitate (23) under thecombined effect of gravity and centrifugal force, and that an exhaustpipe (21) for discharging the precipitate as separated starts from thebottom of the liquid distribution space.
 7. The evaporator according toclaim 6, said evaporator being a film evaporator consisting of heatexchanger elements (3) made of flexible film material.
 8. The evaporatoraccording to claim 6 or 7, wherein the liquid distribution space (14) islocated inside the evaporator jacket (2).
 9. The evaporator according toclaim 6 or 7, wherein the recirculation line (11) is attached to one endof the elongated liquid distribution space (14), and that the exhaustpipe (21) for the precipitate starts from the opposite end of the liquiddistribution space.
 10. The evaporator according to claim 6 or 7,wherein the bottom of the liquid distribution space (14) is slanteddownwards towards the exhaust pipe (21).
 11. The evaporator according toclaim 6 or 7, wherein the liquid distribution space (14) converges in asphenoid or conic form towards the exhaust pipe (21).
 12. The evaporatoraccording to claim 6 or 7, wherein the supply units comprisedistributive nozzles (31) that start from the liquid distribution space(14) and spread out like fans, each one of them feeding solution toseveral parallel gaps between the heat transmission surfaces (4) of theheat exchanger elements (3), evaporation taking place in the gaps. 13.The evaporator according to claim 6 or 7, wherein the trough-like liquiddistribution space (14) is provided with parallel, slanting lamellas(16) between which the solution is allowed to flow upwards.
 14. Theevaporator according to claim 6 or 7, wherein the trough-like liquiddistribution space (14) comprises an intermediate bottom (27) thatdivides it into a lower and upper part (17, 18) that the recirculationline (11) is attached, in the lateral direction, to the lower part (17)of the liquid distribution space, and that the intermediate bottomcomprises ports, through which the solution is allowed to flow to theupper part (18) of the space at the same time as the precipitate (23)ends up in the exhaust pipe (21) that starts from the bottom of thespace.
 15. The evaporator according to claim 14, wherein the flow routesformed by the openings in the intermediate bottom (27) are slantedupstream with regard to the incoming direction of the recirculationflow.
 16. The evaporator according to claim 13, wherein the trough-likeliquid distribution space (14) is provided with a dam plate (15), overwhich the solution flows as an overflow to the supply units (6) of theparallel heat exchanger elements.
 17. The evaporator according to claim6 or 7, wherein the exhaust pipe (21) leads to a settling apparatus(24), which separates the precipitate from the liquid phase that comeswith it, and that the settling apparatus is connected, by using a line(26), to the recirculation line (11), in order to join that separatedliquid phase to the recirculation flow in the evaporator.